In an electric tool such as a bolt tightening machine using an AC motor, a phase control or an antiphase control is carried out over the AC motor so that a tightening torque for a bolt or the like is controlled. In an electric apparatus for carrying out the phase control or the antiphase control, an electromagnetic noise is caused by a rapid change in current in switching. In an electric tool using an AC motor in which a commercial AC power supply is utilized as a power supply, a large current flows through the AC motor. Consequently, there is increased an electromagnetic noise caused by a change in current in switching. For this reason, there is a particular fear that a peripheral electric apparatus or a human body might be adversely influenced.
For example, Japanese Patent Laid-Open Publication No. 2009-12149 and Japanese Patent Laid-Open Publication No. 08-154392 disclose an electric tool for carrying out a phase control of an AC motor using a commercial AC power supply as a power supply by utilizing a triac or an SSR as a switching element. By providing a snubber circuit in the electric tool, it is possible to reduce an electromagnetic noise caused by a change in current. However, there is a problem in that a space for providing the snubber circuit is to be ensured or a cost is increased. By decreasing the current flowing through the AC motor, moreover, it is possible to reduce the electromagnetic noise caused by the change in current. However, a power of the AC motor is reduced. For this reason, the decrease in the current is not preferable in respect of a performance of the electric tool.
In recent years, in the field of power electronics, a transistor capable of controlling a large current is widely used, such as an MOSFET or an IGBT. It is supposed that an electromagnetic noise generated by an electric tool for carrying out a phase control or an antiphase control can be suppressed by using the transistor in place of the triac or the SSR because the use of the transistor is advantageous for a reduction in the change in current in switching. For example, Japanese Patent Laid-Open Publication No. 11-161346 discloses a phase control device for carrying out a phase control or an antiphase control by using two MOSFETs connected in series in opposite directions. It can be proposed that the structure of the phase control device is employed for the electric tool using the AC motor, thereby carrying out the phase control or the antiphase control.